Stretch films are widely used in a variety of bundling and packaging applications. For example, high extensibility machine-applied cast power stretch films (i.e., machine films) are commonly used to secure bulky loads such as boxes, merchandise, produce, equipment, parts, and other similar items on pallets. Typical end-user requirements include high extensibility (greater than 350% elongation at break), resistance to failure (both during and after application), and a high load containment force to the palletized product without the potential for deformation. These properties are all needed while maintaining proper load integrity.
The proper level of containment force applied to the load ensures that the load remains properly secured to the pallet. The “load containment force” is best explained as the residual level of force applied to the load after the film has been allowed to relax for a prescribed length of time. In order to increase the load containment force of a conventional machine film, an end-user has the option to use more film, either by wrapping additional layers of film around a load, or selecting a thicker film. Alternatively, an end-user has the option of stretching the film to a point near its ultimate elongation point. However, stretching a film until it is near its ultimate elongation point imparts high levels of stress and orientation to the film. As a result, the film is vulnerable to defects, abuse, and excessive stretching and is more likely to fail.
Other means have been employed to achieve a higher load containment force without the use of thicker films or additional layers of film. These products typically require the incorporation of either a linear low density polyethylene (LLDPE) of increased modulus (i.e. density) or highly branched, low density polyethylene (LDPE). When either of these methods are employed to increase the load containment force of a film, there is a corresponding decrease in puncture and tear propagation resistance.
Furthermore, metallocene linear low density polyethylene (m-LLDPE) resins are used in producing films. However, m-LLDPE resins with a melt index (MI) of less than 2.0 (g/10 min. @190° C. and 2.16 kg) are typically not compatible for cast power stretch films, due to their lack of elongation and difficulty in processing.
Except for cling purposes on external layers, lower density m-LLDPE resins (less than about 0.915 g/cc) are not typically used in power stretch films, because they lack modulus. Lower density m-LLDPE resins can provide puncture resistance and toughness for a film, but the load holding force is insufficient because they are soft and yield easily. Ziegler-Natta (ZN) based resins below a 0.915 g/cc density are identified by the ultra-low (ULDPE) or very-low (VLDPE) descriptor. The difference between m-LLDPE resin and ZN-LLDPE resin is due to the distribution of the comonomer into the ethylene backbone. Metallocene resins typically have a much narrower molecular weight distribution, and more uniformly insert the comonomer, so at the same density, they have less extractables than their ZN counterparts.
On the other hand, lower melt index m-LLDPE resins can provide load holding force and toughness, but do not have sufficient elongation and are prone to failure.
Next, m-LLDPE with incorporated long chain branching resins can provide increased stretch resistance and failure from tear propagation, but they have relatively poor elongation properties due to the long chain branching (compared to a typical LLDPE which inherently lacks long chain branching) With regards to the long chain branching portion of the polymer, the long branches or arms do not fold or unfold quickly compared to other resins. Therefore, these polymers behave differently under stress and during relaxation.
While separate properties of a film, puncture resistance and tear propagation are interdependent in maintaining the integrity of the palletized load. During the stretching and orienting of the film prior to the application of the film to the load, the film must be resistant to holes and tear propagation in order to be delivered to the palletized load without failure. Even if a hole or tear is present, the film must be designed to resist any significant propagation which would result in film breakage and termination of the process. Even films that are designed to be highly puncture resistant are subjected to tears and holes during and after the wrapping process due to the pallet itself, the product(s) being wrapped, and during the material handling process.
There is, therefore, a long-standing yet unmet need for improved compositions of high performance cast power stretch films with high extensibility, resistance to failure, and load containment force. There is a further unmet need for methods of producing such improved stretch films.